Data encryption is employed to secure data against unauthorized access. For example, data stored in a file system of a computer system can be encrypted such that unauthorized users accessing the file are unable to decrypt the file without one or more requisite cryptographic keys. Whole disk encryption (WDE)—also known as full disk encryption (FDE)—is a technique for encrypting data stored on a data storage disk in ‘rest’ state to protect against access to the disk by an unauthorized user or theft of the disk while it is not in use. Such a disk can include a physical disk or a virtual disk resource provided in a virtualized computing environment. Where a whole disk is encrypted it is difficult for an attacker to make sense of data stored on the disk due to the encryption. Whole disk encryption is one of the most common ways organizations protect their data from theft or loss.
WDE solutions operate on both physical computer systems with physical disks and virtualized computer systems with potentially virtualized data storage disk such as cloud computing platforms. Approaches to WDE are generally based on a “pre-boot” scheme that applies before bootstrapping of an operating system on a physical or virtualized computer system. Specialized encryption software is installed in a portion of a disk used to bootstrap the operating system such as a portion of the disk containing an operating system bootloader. The location of the encryption software varies in different solutions: some solutions employ a Master Boot Record (MBR) as a first sector of a disk; alternative solutions create a dedicated partition and install the encryption software there. For example, the SecureCloud service provided by TREND MICRO requires at least 100 MB of free storage on a boot partition of a disk in order to install its encryption agent software.
The encryption software used in WDE encrypts all partitions selected for encryption on a disk except for that part of the disk storing the encryption software itself (such as an MBR or such dedicated partition). It is not possible for the WDE encryption software to encrypt the part of the disk storing itself because so doing will prevent the computer from bootstrapping since the bootstrap or bootloader itself will be encrypted. Thus in WDE a computer boots from the MBR or un-encrypted partition and the encryption software decrypts and loads the operating system from an encrypted partition.
Accordingly, despite its name, WDE is not actually the encryption of a whole disk where an operating system bootstrap or bootloader is required on the disk. There is therefore a part of a disk that is retained unencrypted so that the encryption software itself can be loaded and executed. This unencrypted portion of the disk constitutes a vulnerability that can be exploited by attacks such as the “bootkit” attack in which an MBR or boot sector of an unencrypted part of a disk is infected with malicious software to tamper with the boot process of a computer, or even to simply passively await a user input of a decryption PIN, key or passphrase and capture such information to access a decryption key that will be loaded in the computer memory.